Surge arresters are used to protect equipment connected to power distribution networks from damage by excessive voltage situations arising from lightning strikes, switching surges, incorrect connections, and other abnormal conditions or malfunctions.
The active element in a surge arrester is a varistor, also referred to as a non-linear resistor because it exhibits a nonlinear current-voltage relationship. If the applied voltage is less than a certain voltage (the switching or clamping voltage) the varistor is essentially an insulator and only a small leakage current flows through it. If the applied voltage is greater than the switching voltage, the varistor resistance drops, allowing an increased current to flow through it. That is, a varistor is highly resistive below its switching voltage and substantially conductive above it. Exemplary surge arrester designs are disclosed in Honl et al., U.S. Pat. No. 5,299,088 (1994); Koch et al., EP 0,230,103 A2 (1987); Koch et al., EP 0,229,464 A1 (1987); Mikli, U.S. Pat. No. 5,325,087 (1994); and Wiseman et al., WO 93/26017 (1993).
The surge arrester is commonly attached to an electrical power system in a parallel configuration, with one terminal of the device connected to a phase conductor of the electrical power system and the other terminal to ground or neutral. At normal system voltages, the surge arrester is resistant to current flow (except for the leakage current). But if an overvoltage condition exceeding the switching voltage develops, the surge arrester becomes conductive and shunts the surge energy to ground while "clamping" or limiting the system voltage to a value which can be tolerated without damage by the equipment being protected.
In the event of a persistent power frequency overvoltage, the volt-time withstand of equipment insulation can exceed the damage limit, resulting in failure. While arresters prevent such damage for relatively short periods of time for lightning surges (tens to hundreds of microseconds) or switching surges (thousands of microseconds), a sustained or permanent power-frequency overvoltage condition, even with varistor voltage limitation, can result in equipment insulation damage and/or failure. While the intent of a typical surge arrester is to limit temporary overvoltage conditions, it is not intended to protect against power frequency sustained or permanent conditions. It is desirable for the surge arrester, under such power frequency overvoltage conditions, to create a permanent, low resistance path to ground, in order to create an intentional short-circuit which will result in the intentional operation of overcurrent protection, which will sectionalize and deenergize the arrester and the protected equipment in order to avoid an otherwise potentially damaging condition.